1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic latent image, a method of producing such a toner, a developer, an image-forming device and an image-forming method which are used for electrophotography, and an electrostatic recording method.
2. Description of the Related Art
In electrophotography, an electrostatic latent image, which is formed on a latent image holding member (photoreceptor), is developed with a toner containing a colorant, the obtained toner image is transferred onto a transfer material and fixed by a heat roller or the like to create an image, and the latent image holding member is cleaned so that another electrostatic latent image can be formed. Dry developers used for the electrophotography or the like can be broadly divided into single-component developers which solely use a toner having a colorant or the like mixed with a binder resin and two component developers having a carrier mixed with a toner as described. Single-component developers may in turn be classified into those comprising single magnetic components having a magnetic powder and are transported for development to a developer holding member by magnetic force, and those comprising a single nonmagnetic component not having magnetic powder and which are transported to a developer holding member for development after being charged by a charging roller or the like.
Since around the latter half of the 1980's, there has been a great demand for reduction in size and increased performance for digitalization of the electrophotography market, and particularly a demand for full-color images of a quality similar to that of high-grade printing and silver-salt film photos.
Digitalization processing is necessary to achieve high image quality. Image digitalization in turn requires complex image processing at a high speed. With digitalization, it becomes possible to separately control characters and photo images, and reproducibility and quality of both has improved remarkably as compared with concurrent improvements in analog technology. Especially, the improvement is remarkable in that it has become possible to make gray level correction and color correction of a photo image, and it is advantageous over the analog technology in terms of a gradation property, fineness, sharpness, color reproduction, and graininess. However, output of an image requires forming the image faithfully from a latent image which is formed by the optical system. Meanwhile, although toner particle size has become smaller, and studies to provide accurate reproducibility have been promoted, it has not proved possible to obtain reliable high image quality simply by reducing the size of toner particles. Corresponding improvement in the basic properties of the developing, transfer, and fixing properties is also necessary.
Especially in color imaging, in which a color image is formed by superposing three or four colored toners, if any one of the toners does not perform as expected in terms of developing, transfer, and fixing properties, or performs different from the other colors, degradation of image quality such as deterioration of color reproduction, deterioration of graininess, uneven color, or the like may occur. To maintain an image having the same stable high quality as the initial quality after a lapse of time, it is important to stably control the properties of individual colors.
Japanese Patent Application Laid-Open (JP-A) No. Hei 10-312089 discloses that the toner is stirred in the developing unit, a fine structure of the toner surface is easily varied, and transferability is considerably changed.
Lately, for miniaturization of the device to save space, reduction of toner waste for environmental protection, and elongation of the service life of the latent image holding member, there is proposed a cleaner-free system in, for example, JP-A No. Hei 5-94113. The system disclosed in that publication does not have a cleaning system, the toner remaining on the photosensitive drum after image transfer is dispersed by a brush which is in contact with the photosensitive drum, and the dispersed toner is simultaneously recovered by the developing unit during the developing phase. Generally, when the residual toner is recovered during developing, there is a disadvantage that the recovered toner and the other toner have different electrostatic properties, and the recovered toner is normally not used for developing but is instead accumulated in the developing unit. It therefore remains necessary to improve the transfer efficiency so to control to minimize the amount of toner to be recovered.
JP-A No. Sho 62-184469 proposes use of a toner having a spherical shape in order to improve flowability, electrostatic properties, and transferability. However, when the toner has a spherical shape, certain disadvantages result. The developing unit is provided with a conveying amount control panel for controlling a conveying developer amount to a prescribed level. This control can be effected by varying a space between the magnetic roller and the conveying amount control panel. However, when a spherical toner is used, along with improvement in flowability of the developer, the set bulk density also becomes high. As a result, developer puddles are formed on the conveyance control section, and there is a phenomenon that a conveying amount becomes instable. Although the conveying amount can be improved by controlling the surface roughness of the magnetic roller and also by narrowing the space between the control panel and the magnetic roller, the packing property is enhanced because of the developer puddles, and an increasing stress is applied to the toner. Thus, the fine structure of the toner surface changes easily, and particularly burying or peeling of an external additive easily occurs. Thus, it is confirmed that there is a disadvantage that development and transferability become largely different from the initial ones.
In an attempt to address such disadvantages, JP-A No. Hei 6-308759 discloses that a superior image quality can be achieved by suppressing the packing property by combining a spherical toner and a non-spherical toner. Although the disclosed method is effective in terms of suppression of the packing property, because the non-spherical toner tends to remain as untransferred toner, high transfer efficiency cannot be achieved. When developing and recovery are performed simultaneously, the composite ratio of the non-spherical toner increases because a disproportionate amount of the untransferred non-spherical toner is recovered, and transfer efficiency is steadily lowered.
To improve the developability, transferability, and cleanability of the spherical toner, JP-A No. Hei 3-100661 discloses that two types of inorganic fine particles having different particle sizes, specifically particles having an average particle size of 5 mμ or more and less than 20 mμ and particles having an average particle size of 20 mμ or more and 40 mμ or less, are used together and added in prescribed amounts. Although initially such a combination of particles provides high developability, transferability, and cleanability, because the force applied to the toners over time cannot be reduced, burying or peeling of the external additive occurs, and development and transferability can greatly vary from initial characteristics.
Meanwhile, JP-A No. Hei 7-28276, JP-A No. Hei 9-319134, and JP-A No. Hei 10-312089 disclose that use of inorganic fine particles having a large particle size can effectively suppress burying of the external additive in the toner as a result of such stress.
In the above publications, because the inorganic fine particles have large specific gravities, peeling or the like of the external additive cannot be avoided because of a stirring stress within the developing unit when the external additive particles become large. Also, when the inorganic fine particles adhere to the toner surface, maintaining a prescribed level of bead chains of the external additive to becomes difficult because the inorganic fine particles do not have a perfect spherical shape. Thus, the shapes of microscopically protruding surfaces serving as spacers vary and selectively suffer from a stress on their protruded portions, so that burying or peeling of the external additive is further accelerated and the spacer effect is insufficient. In order to avoid such disadvantages, JP-A No. 2001-66820 discloses use of monodisperse spherical silica having a low specific gravity. Although spacer effects can be obtained when such silica is used, defects are created in the organic photoreceptor surface due to abrasion of silica when a mechanical pressure is produced during cleaning by the blade, or by the electrostatic brush after transfer, or produced because of a peripheral speed different from a photoreceptor when a charging roller is used for a contact type charger. The number and size of defects increases over time, with the result that the incidence of defective images tends to increase as the device is used.
JP-A No. Hei 6-266152 discloses a technology for adding organic fine particles of 50 to 200 nm to the toner to produce an effective spacer capability. Initially effective spacer capability can be created through use of spherical organic fine particles. The organic fine particles are not susceptible to burying or peeling by stress over time, but do not maintain a high spacer capability with stability because the organic fine particles disclosed in this publication are deformed. The spacer effect may be obtained by adhering a large amount of organic fine particles to the toner surface or by using the organic fine particles having a large particle size. When this is done, the properties of the organic fine particles are largely reflected. Specifically, effects on the powder properties, such as inhibition of flowability and a deterioration of heat cohesion of the toner with inorganic fine particles added and effects on charging and development that a degree of flexibility to control in terms of charging, are impaired because the organic fine particles themselves have charge-imparting capability. To compensate for the disadvantage that the organic fine particles themselves are deformed, there has been proposed resin fine particles having Vickers hardness of 3 to 50 kg/mm and a flat particle size of 0.03 to 1.0 μm in JP-A No. Hei 4-274442, a melamine-based resin in JP-A No. Hei 4-328757, particular crosslinking organic fine particles in JP-A No. Hei 6-11883, organic and inorganic composite particles in JP-A No. Hei 9-194593, JP-A No. Hei 9-197705 and JP-A No. Hei 9-197706, organic fine particles of a substantially spherical cross-linked copolymer of 0.2 μm or less in JP-A No. Hei 11-338183, and crosslinking organic fine particles having a Cv value of particle size of 20% or less and a gel ratio of 25% or more in JP-A No. 2001-163985.
The above patent publications JP-A No. Hei 4-274442, JP-A No. Hei 4-328757, JP-A No. Hei 6-11883, JP-A No. Hei 9-194593, JP-A No. Hei 9-197705, JP-A No. Hei 9-197706 and JP-A No. Hei 11-338183 describe improvements to the transferability of the particles to some extent, but all require the addition of an excessively large amount of particles to the toner because grain size distribution of particles is extensive. Thus, the above-described charging and development are greatly affected, and, when the amount of added particles is reduced, sufficient transferability cannot be obtained. Because the particles in the above references have a small electrostatic property, the melamine-based resin particles of JP-A No. Hei 4-328757 specifically having a positive electrostatic property, they adversely affect imaging because they cause a charging level of the negative charged toner to fall precipitously. Because the particles of JP-A No. 2001-163985 have hardness to some extent and a narrow grain size distribution, transferability is improved when a relatively small amount of the particles are added, but there is still an adverse effect that charging is small.
There is now a great demand for colorizing and an even greater demand for on-demand printing, and there is reported a method in which multi-color images are formed on a transfer belt to copy multiple sheets at a high speed and transferred and fixed onto an image fixing material by a single operation (e.g., JP-A No. Hei 8-115007). When it is assumed that a step of transferring from the photoreceptor to the transfer belt is a primary transfer and a step of transferring from the transfer belt to the transfer material is a secondary transfer, the transfer is repeated twice, and the significance of a technology for improving transfer efficiency grows more important. Because the secondary transfer involves the transfer of multiple color images in a single operation, with the properties of the transfer material (e.g., paper thickness, surface property, etc.) being variable to a certain extent, it is necessary to tightly control the charging, developing, and transferring in order to reduce the influence of the transfer material on the resulting image.
To reduce power consumption and space and to produce higher quality images, there are disclosed technologies of simultaneously transferring and fixing colors onto the intermediate transfer material (e.g., JP-A No. Hei 10-213977 and JP-A No. Hei 8-44220). In this regard, it is important that the transfer belt is provided with transfer capability and fixing capability. Specifically, a thin belt made of a belt material having high heat resistance is used because it is necessary to improve transferability at the primary transfer portion in a cooled state and to instantaneously conduct heat at the secondary transfer and fixing portion. The toner must be fixed under a low pressure because transfer efficiency cannot be controlled to a very high level and a high pressure cannot be applied at the time of fixing. It is also important that contamination caused by the toner when fixing or by an external additive or the like be minimized because the belt surface also has a transfer function.
Meanwhile, there are also proposed methods of faithfully reproducing high image quality, and particularly a half tone, a solid black area or characters by controlling volume specific resistance of the carrier (e.g., JP-A No. Sho 56-125751, JP-A No. Sho 62-267766, Japanese Patent Publication No. Hei 7-120086). Such methods adjust the resistance according to a type of carrier-coated layer or a coated amount and can initially provide a target volume specific resistance and develop high image quality. However, the carrier-coated layer suffers from susceptibility to peeling or the like, and volume specific resistance varies greatly, because of stress in the developing unit. It is therefore difficult to maintain high image quality over a long period of time.
There is also proposed a method of adjusting volume specific resistance by adding carbon black to the carrier-coated layer (e.g., JP-A No. Hei 4-40471). Although this method can effectively suppress changes in volume specific resistance caused by peeling of the coated layer, the external additive added to the toner or the toner components adheres to the carrier, causing changes in the volume specific resistance of the carrier, and therefore impairing expression of high image quality over a long period, as is the case of the above-described carrier.